This application is related to commonly-assigned U.S. patent application Ser. No. 12/969,861, filed Dec. 16, 2010; U.S. patent application Ser. No. 12/969,876, filed Dec. 16, 2010; and U.S. patent application Ser. No. 12/969,906, filed Dec. 16, 2010.
The present invention relates generally to turbomachines and more particularly to a method for enhancing the operational flexibility of a steam turbine during a shutdown phase.
Steam turbines are commonly used in power plants, heat generation systems, marine propulsion systems, and other heat and power applications. Steam turbines typically include at least one section that operates within a pre-determined pressure range. This may include: a high-pressure (HP) section; and a reheat or intermediate pressure (IP) section. The rotating elements housed within these sections are commonly mounted on an axial shaft. Generally, control valves and intercept valves control steam flow through the HP and the IP sections, respectively.
The normal operation of a steam turbine includes three distinct phases; which are startup, loading, and shutdown. The startup phase may be considered the operational phase beginning in which the rotating elements begin to roll until steam is flowing through all sections. Generally, the startup phase does not end at a specific load. The loading phase may be considered the operational phase in which the quantity of steam entering the sections is increased until the output of the steam turbine is approximately a desired load; such as, but not limiting to, the rated load. The shutdown phase may be considered the operational phase in which the steam turbine load is reduced, and steam flow into each section is gradually stopped and the rotor, upon which the rotating elements are mounted, is slowed to a turning gear speed.
The shutdown phase for steam turbines equipped with cascade steam bypass systems may impose unique operational characteristics, which may overload the thrust bearings. A conventional shutdown strategy can involve a flow-balancing process that balances flow between the HP and IP sections until a HP forward flow mode ends. Forward flow may be considered steam flowing, in a forward direction, through the HP section. During HP forward flow mode, steam flow through the HP and IP sections is fairly balanced. Here, the flow rate typically depends on the operating reheat (RH) pressure.
There are a few drawbacks with the conventional shutdown strategy. Flow-balancing strategies may not effectively manage competing physical requirements. Here, a single physical requirement or parameter can limit the operation of the entire steam turbine. Furthermore, determining when to terminate the HP forward flow mode may be an issue. If the HP forward flow mode is terminated early in the shutdown process, the resulting high flow rate may increase the thrust load. If the HP forward flow mode is terminated later in the shutdown process, undesirably high HP section exhaust temperatures may result, possibly due to RH pressure issues.
These issues reduce the operational flexibility, require larger mechanical components, and potentially reduce the net-output delivered by the steam turbine during the shutdown phase. Therefore, there is a desire for a method for increasing the operational flexibility of the steam turbine during the shutdown phase.